Biodiesel fuel is frequently made from triglycerides derived from animal or plant sources. The triglyceride is reacted with an alcohol, typically, methanol in the presence of a catalyst (typically sodium methoxide) in a transesterification reaction to produce fatty acid alkyl esters (biodiesel) and glycerin (a byproduct). The ester fraction is separated from the glycerin layer (e.g., after catalyst neutralization with an acid like phosphoric acid) and then purified using water washing and distillation to decrease the levels of impurities and pass a specification under ASTM D6751 indicative of the availability of the ester for use in colder temperatures. The processed ester is also known as biodiesel. The cold temperature test commonly used today is the Cold Soak Filtration Test (CSFT) as described by ASTM D7501, which is used to measure the product's “cold flow properties”.
The cold flow properties of biodiesel depend on the nature of the fatty acid chains of the feedstock (paraffinic versus ° Jennie), as well as the presence of impurities such as monoglycerides (intermediate product in the transesterification reaction) and sterol glycosides (originating from the feedstock) remaining after the purification steps. Sterol glycosides are compounds present in ppm quantities in natural oils. They are typically present in the acylated form which is soluble in the oil. During the transesterification process, these compounds get converted to the non-acylated form of sterol glycosides which have low solubility in the fatty acid alkyl esters. It has been shown that the presence of saturated monoglycerides and the non-acylated sterol glycosides (both with relatively high melting points >40° C.) can cause significant clouding of the biodiesel and result in clogging of fuel filters, especially under cold weather conditions. This phenomenon has been observed in streams containing 100% biodiesel (also referred to as “B100”), as well as streams comprising blends of biodiesel and petroleum derived diesel (also referenced as “BXX” blends, in which the XX is a number reflecting the percentage of biodiesel in the blend). To overcome these problems, the aforementioned ASTM D7501 tests have been implemented in the United States to qualify fatty acid alkyl esters for biodiesel for cold weather performance.
Current methods for purifying fatty acid alkyl ester streams involve low temperature adsorbent approaches, wherein adsorbents bind or otherwise remove impurities from the desired fatty acid alkyl ester. For example:                (1) WO 2009/099655 discloses a cartridge-based solution to process ester product that meets the CSFT. This process comprises two steps for continuous purification of biodiesel, one of which includes the use of an adsorbent or filter-aid.        (2) US 2010/0313468 describes a process of treating biofuels with an adsorbent comprising two different metal oxides including zeolites, silicas, aluminas or combinations thereof.        (3) US 2007/0151146 describes a process utilizing a silica hydrogel, among other samples, for alleviating the “filter-blocking tendency” of biodiesel.        (4) US 2009/0156847 discloses using resin-based adsorbents for biodiesel purification.        (5) U.S. Pat. No. 7,635,398 describes a magnesium silicate product that can remove biodiesel impurities and help pass the CSI-T.        (6) WO 2009/132670 describes clay adsorbents for processing product to meet the CSFT requirement.        
Adsorbent processes, however, require frequent regeneration to remove bound species from binding sites, or removal/replacement of the adsorbent to introduce fresh adsorbent. Regeneration in continuous processes, moreover, require either shutting down the system and flushing it with solvent to regenerate the adsorbents, or requiring at least a second adsorbent column containing fresh adsorbent through which the process stream is run while the other column is being regenerated. Distillation processes require capital investment at a significant cost.
It therefore is desirable to have alternative processes, especially those that can be run with less frequent changeover or regeneration of material used to purify the esters, with a lower capital investment cost relative to distillation, as well as have an approach that can be widely adopted regardless of the specific process used to manufacture the fatty acid alkyl ester.